Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. Cellulose is especially abundant in tree trunks from which it is extracted, converted into pulp, and thereafter utilized to manufacture a variety of products. Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. This is reacted with carbon disulfide to form cellulose xanthate which is then dissolved in dilute caustic soda solution. After filtration and deaeration the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting so-called viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammonia copper oxide. This property forms the basis for production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents that were used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of underivatized cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine-N oxides, in particular the tertiary amine-N oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a fiber composed of cellulose precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Acordis, Ltd. presently manufactures and sells a lyocell fiber called Tencel.RTM. fiber.
It is believed that currently available lyocell fibers are produced from high quality wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose. These highly processed pulps are referred to as dissolving grade or high alpha (or high .alpha.) pulps, where the term alpha (or .alpha.) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose. The processing required to generate a high alpha pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
For example, when the Kraft process is used to produce a dissolving grade pulp, a mixture of sodium sulfide and sodium hydroxide is used to pulp the wood. Since conventional Kraft processes stabilize residual hemicelluloses against further alkaline attack, it is not possible to obtain acceptable quality dissolving pulps, i.e., high alpha pulps, through subsequent treatment of Kraft pulp in the bleaching stages. In order to prepare dissolving type pulps by the Kraft process, it is necessary to give the raw material an acidic pretreatment before the alkaline pulping stage. A significant amount of material primarily hemicellulose, on the order of 10% or greater of the original wood substance, is solubilized in this acid phase pretreatment and thus process yields drop. Under the prehydrolysis conditions, the cellulose is largely resistant to attack, but the residual hemicelluloses are degraded to a much shorter chain length and can therefore be removed to a large extent in the subsequent Kraft cook by a variety of hemicellulose hydrolysis reactions or by dissolution.
The prehydrolysis stage normally involves treatment of wood at elevated temperature (150-180.degree. C.) with dilute mineral acid (sulfuric or aqueous sulfur dioxide) or with water alone requiring times up to 2 hours at the lower temperatures. In the latter case, liberated acetic acid from certain of the naturally occurring polysaccharides (predominantly the mannans in softwoods and the xylan in hardwoods) lowers the pH below 4.
Moreover, a relatively low copper number, reflective of the relative carbonyl content of the cellulose, is a desirable property of a pulp that is to be used to make lyocell fibers because it is generally believed that a high copper number causes cellulose and solvent degradation, before, during, and/or after dissolution in an amine oxide solvent. The degraded solvent can either be disposed of or regenerated; however, due to its cost it is generally undesirable to dispose of the solvent. Regeneration of the solvent suffers from the drawback that the regeneration process involves dangerous, potentially explosive conditions.
A low transition metal content is a desirable property of a pulp that is to be used to make lyocell fibers because, for example, transition metals accelerate the undesirable degradation of cellulose and NMMO in the lyocell process.
In view of the expense of producing commercial dissolving grade pulps it would be desirable to have alternatives to conventional high alpha dissolving grade pulps as a lyocell raw material. In addition, pulp manufacturers would like to minimize the capital investment necessary to produce such types of pulps by utilizing existing capital plants.
In order to control lyocell fiber properties, lyocell manufacturers utilize dopes that comprise a blend of different pulps having different ranges of average degree of polymerization values. In view of this, there is also a need for pulp manufacturers to produce pulps having an average degree of polymerization within a relatively narrow band.
Thus, there is a need for relatively inexpensive, low alpha (e.g., high yield) pulps that can be used to make lyocell fibers, for a process of making the foregoing low alpha pulps using capital equipment that is currently available to pulp manufacturers, and for lyocell fibers from the foregoing low alpha pulp. Preferably, the desired low alpha pulps will have a desirably low copper number, a desirably low lignin content and a desirably low transition metal content.
In the prior application Ser. No. 09/256,197, now U.S. Pat. No. 6,210,801 B1, assigned to the assignee of the subject application, various methods of reducing D.P. values and copper number of a Kraft pulp are described. Such methods include treating pulp with acid, or an acid substitute, or a combination of acids and acid substitutes. Other means of treating the pulp to reduce the average D.P. of cellulose without substantially reducing the hemicellulose content described in the prior application include treatment of the pulp with steam, a combination of ferrous sulfate and hydrogen peroxide, at least one transition metal and peracetic acid, an alkaline chlorine dioxide treatment which ends acidic or a sodium hypochlorite treatment which ends near neutral. Such processes are effective at reducing the average degree of polymerization without substantially reducing the hemicellulose content, however, such processes can be expensive from a capital improvement standpoint if the existing pulp mills in which such processes are to be used are not configured to allow for the simple deployment of such processes. In the prior application, additional steps are described in order to reduce the copper number of the pulp which have been treated to reduce its average degree of polymerization without substantially decreasing the hemicellulose content. The need for this subsequent copper number reducing step arose because the methods described in the prior application for reducing the average degree of polymerization for the cellulose resulted in an increase in the copper number for the resultant pulp.
In view of environmental concerns, there has been a great interest in using bleaching agents, which reduce the amount of chlorocompounds that must be recovered from process streams. In recent years, the use of oxygen as a delignifying agent has occurred on a commercial scale. Examples of equipment and apparatus useful for carrying out an oxygen stage delignification are described in U.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and 4,295,926.
While the methods described in the prior application are effective at reducing the average D.P. of cellulose without substantially decreasing the hemicellulose content, a need exists for a process that does not require a separate copper number reducing step and which is readily adaptable to pulp mills that include oxygen reactors, multiple alkaline stages and/or alkaline conditions suitable for substantial D.P. reduction of bleached or semi-bleached pulp.